/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/MeshOpPreviewHelpers.h
Configure the In-Progress and Secondary Materials
void ConfigurePreviewMaterials ( [UMaterialInterface](API\Runtime\Engine\Materials\UMaterialInterface) * InProgressMaterial, [UMaterialInterface](API\Runtime\Engine\Materials\UMaterialInterface) * SecondaryMaterial )